A current, known aerial image sensor (AIS) system senses an aerial image by projecting a slit from a reticle to a slit (referred to as the substrate side slit) on a metrology stage on the substrate-side of a projection lens. A collection lens (also referred to as a receiving lens) is used to collect the radiation leaving the substrate-side slit and relays it to a single detector element. As the substrate-side slit is scanned relative to the image of the reticle-side slit, certain information about the projection lens can be deduced from the detector signal.
One issue with the current MS system relates to its ability to collect radiation from the full numerical aperture (NA) of the system. Ideally the collection lens would be able to collect not only the entire NA, but also the majority of the radiation diffracted from the substrate-side slit. And, it is desirable that the collection lens be simple and reduced in weight—particularly if the collection lens were to be positioned on a substrate stage rather than a separate metrology stage, since any additionally bulk and weight could translate into a reduction in throughput by slowing the stage.